A display has been actively developed, in which a self-light emitting type thin film light emitting element that emits light by itself by applying current thereto is used.
In the thin film light emitting element, an electrode is connected to a single layer- or multilayer-thin film that is formed by using an organic compound, inorganic compound or both thereof, and current is applied, so that light is emitted. As for the thin film light emitting element, lower power consumption, space-saving and visibility are expected. Moreover, the market is also expected to be expanded in the future.
Among them, as for the light emitting element that has a multilayer structure, it has become possible to form an element that can emit light efficiently by separating functions per layer as compared with the conventional one (see C. W Tan, et al., Applied Physics. Letters, Vol. 51, No. 12, 913-915 (1987), for example).
A thin film light emitting element that has a multilayer structure includes a light-emitting lamination body that is interposed between an anode and a cathode, and the light-emitting lamination body is formed by a hole injecting layer, a hole transporting layer, a light-emitting layer, an electron transporting layer, an electron injecting layer or the like. Not all of the hole injecting layer, the hole transporting layer, the electron transporting layer and the electron injecting layer may be provided, depending on the element structure. In addition, the hole transporting layer and the electron transporting layer sometimes serve as the light-emitting layer. In this case, a method of doping a highly carrier transporting-electron transporting layer or hole transporting layer with a pigment that has high luminous efficiency is used. By employing this method, a material that has high luminous efficiency but lower carrier transporting property can be used as a luminescent substance (see C. W. Tan et al., Journal of Applied Physics, Vol. 65, No. 9, 3710-3716 (1989), for example).
Luminescence is obtained by applying current to the thin film light emitting element, and there exists proportionality between luminance of the luminescence and the current that is applied. That is to say, large current has to be applied to obtain high luminance. Therefore, it is attempted that current be applied easily and drive voltage be reduced by doping at least a part of thin films that are laminated between the electrodes of the light emitting element with additives. In addition, in this case, comparatively low voltage is necessary to obtain the same luminance; thus, deterioration of the thin film light emitting element is suppressed (see Japanese Patent Application Laid-Open No: 2003-77676, for example).
In JP '676, a substance containing a polycyclic condensed ring such as diphenylanthracene or rubrene in which its carrier transporting property is relatively favorable is used as an additive. Advantages such as lower drive voltage are obtained by doping the electron transporting layer with the substance mentioned above.
However, the substances containing a polycyclic condensed ring that are preferably used as an additive usually emit light by themselves. Therefore, when an energy gap of a luminescent substance in the light emitting layer is larger than that of the additive, energy of the excited luminescent substance is transferred to the additives that are added into the electron transporting layer; thus, the additives can also be excited to emit light. In addition, luminous efficiency can be lowered by energy transfer even when the additives do not emit light.
As described above, when a substance emitting light exists in addition to the luminescent substance in the light emitting layer, color purity is deteriorated, which is not preferable. Therefore, when additives are used, the additives that have a larger energy gap than that of the luminescent substance in the light emitting layer are usually used. In Embodiment 1 of JP '676, diphenylanthracene that has a larger energy gap than that of rubrene added as a luminescent substance in a light emitting layer is used as an additive.
In the case of a luminescent substance that emits red light, additives that fulfill the requirements are easily selected since it has a small energy gap. However, when luminescence from a luminescent substance that has a larger energy gap is required, choices are more and more limited in the current situation. For reference, the rubrene shows favorable electron injecting property and electron transporting property by doping the electron injecting layer and the electron transporting layer with the appropriate amount thereof. However, the luminescent color is yellow so that rubrene is not preferably used for the light emitting element that has the light emitting layer using the luminescent substance having a larger energy gap than that of rubrene (it can be thought to be a substance that emits from green light to purple light).
It is an object of the present invention to provide a thin film light emitting element in which drive voltage is low. In addition, it is also an object of the present invention to provide a thin film light emitting element which has a structure of low drive voltage and in which color purity and luminous efficiency are not deteriorated.